"We've been working for years on doing more and more in the United States," Cook said.

"The consumer electronics world was really never here... It's a matter of starting it here."

Apple has faced mounting criticism for the labour standards of companies like Foxconn, through which it manufactures the bulk of its products in China and other Asian countries.

Earlier this year Apple hired the Fair Labor Association in the US to examine those conditions, after reports of young workers being employed illegally, and poor conditions.

Recently it emerged that some of Apple's computers now say "assembled in the USA" on their casing, unlike the usual "assembled in China".

Cook said Apple would not build factories themselves but said it would invest $100 million to get the work done.

We could have quickly maybe done just assembly, but it’s broader because we wanted to do something more substantial. So we’ll literally invest over $100 million. This doesn’t mean that Apple will do it ourselves, but we’ll be working with people, and we’ll be investing our money.

But Cook said it would not be easy to bring manufacturing jobs back to the US, partly because US workers didn't have the right skills.